1. Field of the Invention.
This invention relates to an improved trailer braking system and, more specifically, to such a system which utilizes a small control air reservoir on the trailer to provide a reliable secondary source of control air to a control valve for quickly and safely applying the trailer brakes prior to the arrival of normal control air from the tractor.
2. Description of the Prior Art.
As commerical tractor-trailer configurations are getting longer, in some cases including multiple trailers, and are operating on more congested highways, it is essential to ensure that the trailer braking systems operate properly for safe, reliable stopping. It has been found in many conventional tractor-trailer braking configurations that a time delay in the application of the trailer brakes has been experienced.
Normally, the tractor will include a primary air reservoir which is provided pressurized air by a compressor or the like which is driven by the vehicle engine. The primary air reservoir supplies pressurized air through piping to an auxiliary air reservoir located near the trailer axle. Additionally, the primary air reservoir is a source of air for a foot pedal actuator valve to provide control air for operating the tractor brakes and the trailer brakes. However, because of the length of the line extending from the actuator valve to the rearward portion of the trailer and the number of flow impediments in the line, there exists a significant time delay in the travel of the control air to the trailer brakes. Under an ideal system, air pressure would travel through clean, frictionless tubing at about the speed of sound. However, since the control air line includes valves, couplings, fittings and bends, a time delay of one-half a second or greater is not uncommon. With such a time delay, the brakes on the tractor are applied before those on the trailer. As a result, the tractor-trailer could experience problems during braking including the possibility of increased stopping distances or jackknifing.
In an effort to eliminate this problem, U.S. Pat. No. 3,497,268 diclosed the use of a small air tank located near the auxiliary air reservoir as an alternative source of control air for the operation of the trailer brakes. Operation of the small air tank is controlled by an electrical signal, such as the brake light signal, when the brake pedal on the tractor is depressed. The electrical signal is sent through wiring from the tractor to the trailer at the speed of light to eliminate the time delay which is experienced in the conventional system when the control air must travel the same distance. Specifically, the auxiliary air reservoir is coupled through a normally open, electrically operated valve to the small tank. The small tank is then coupled through a normally closed, electrically operated valve to one side of a two-way check valve. Normal control air from the tractor is supplied to the other side of the two-way check valve. The outlet of the two-way check valve is coupled to a relay valve, or control valve, for operation of the trailer brakes. Specifically, the relay valve receives a signal in the form of control air from either the tractor or a small air tank to, in turn, direct operating air from the auxiliary air reservoir to the trailer brake operating chambers. Depressing the foot pedal would send the electrical signal to close the normally open valve and to open the normally closed valve. It is assumed that the two-way check valve was to be used to insure that control air from the small air tank would be positively directed to the relay valve. However, there is some doubt that the system as disclosed in the U.S. Pat. No. 3,497,268 would function properly. The effective use of a two-way check valve as disclosed is highly questionable since such a valve arrangement would appear to prevent the release of control air from the relay valve or operating air from the trailer brake operating chambers which is essential for their proper operation.
It is also significant that the use of two electrically operated solenoid valves leaves the trailer braking system vulnerable to a malfunction which could result in an unsafe braking condition. Specifically, if the electrical wire going to the the normally open, electrically operated valve were to be broken, the electrical signal from the brake pedal would not function as intended to only supply of control air from the small air tank. Instead, both valves would be open at the same time so that air directly from the auxiliary air reservoir would be supplied to the relay valve. A similiar result would occur if a ground wire from the normally open, electrically operated valve were broken. Consequently, the operator would find that any movement of the brake pedal would produce a rapid, full braking force on the trailer rather than the gradual, adjustable braking force as normally required for safe vehicle operation.
It should be noted that the practical valve of any system which would improve the braking characteristics of a tractor-trailer configuration would be significantly improved if the system could be readily and simply employed without the requirement of any changes in the tractor electrical circuiting or braking system. There is some question whether the system shown in U.S. Pat. No. 3,497,268 could meet this requirement. There are standard tractor-trailer electrical plugs that must be used for proper interchange of tractors and trailers and the plugs presently operate near capacity. It is possible that the electrical current requirements for two solenoids would exceed that capacity.